DMLC motion tracking of moving targets for intensity modulated arc therapy treatment - a feasibility study

Purpose. Intensity modulated arc therapy offers great advantages with the capability of delivering a fast and highly conformal treatment. However, moving targets represent a major challenge. By monitoring a moving target it is possible to make the beam follow the motion, shaped by a Dynamic MLC (DMLC). The aim of this work was to evaluate the dose delivered to moving targets using the RapidArcTM (Varian Medical Systems, Inc.) technology with and without a DMLC tracking algorithm. Material and methods. A Varian Clinac iX was equipped with a preclinical RapidArcTM and a 3D DMLC tracking application. A motion platform was placed on the couch, with the detectors on top: a PTW seven29 and a Scandidos Delta4. One lung plan and one prostate plan were delivered. Motion was monitored using a Real-time Position Management (RPM) system. Reference measurements were performed for both plans with both detectors at state (0) "static, no tracking". Comparing measurements were made at state (1) "motion, no tracking" and state (2) "motion, tracking". Results. Gamma analysis showed a significant improvement from measurements of state (1) to measurements of state (2) compared to the state (0) measurements: Lung plan; from 87 to 97% pass. Prostate plan; from 81 to 88% pass. Sub-beam information gave a much reduced pattern of periodically spatial deviating dose points for state (2) than for state (1). Iso-dose curve comparisons showed a slightly better agreement between state (0) and state (2) than between state (0) and state (1). Conclusions. DMLC tracking together with RapidArcTM make a feasible combination and is capable of improving the dose distribution delivered to a moving target. It seems to be of importance to minimize noise influencing the tracking, to gain the full benefit from the application.